The club will be without Victor Rodriguez, who is out of the country for the birth of a child.

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A first step toward defending their MLS Cup title can be taken by the Sounders on Sunday in Philadelphia.

The Sounders can potentially clinch a playoff spot and improve their chances of a Western Conference title with a road win against the sub-.500 Union. Coming off a huge victory over Vancouver on Wednesday night — their first win in a month — the Sounders should be a confident bunch heading into their third match in eight days.

“They’re reinvigorated after that performance,’’ Sounders coach Brian Schmetzer said. “That performance did a lot, obviously, for the guys that scored goals. But it was another clean sheet for (goalkeeper) Stef (Frei), so I think overall, the team’s in a good place.’’

Sunday

Sounders FC @ Philadelphia, 10 a.m., ESPN

Frei made a spectacular leaping save off a Fredy Montero free kick in the 37th minute when the game was still up for grabs.

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One of the goal-scorers from Wednesday’s 3-0 win over the conference-leading Whitecaps, midfielder Victor Rodriguez, is back home in Barcelona with his wife awaiting the birth of a child. Seeing Rodriguez — who also had an assist — click with his new teammates in Wednesday’s game was a welcome sign for a Sounders team that had scored just three goals in its previous five matches.

But even without Rodriguez, this is a game the Sounders must capitalize on against a 9-13-9 squad also playing its third match in eight days and on the verge of being eliminated from playoff contention. The Sounders’ back line did a strong job of shutting down Montero and the high-powered Vancouver attack and could buy the Sounders time to put some balls in the net on Sunday.

“The team’s been playing well defensively all year,’’ Schmetzer said.

Right back Kelvin Leerdam felt the Sounders “answered very good’’ against Vancouver after its 13-game unbeaten streak had been ended by Real Salt Lake the previous weekend. Leerdam felt the communication between defenders was better and needs to carry over into Sunday’s match.

“It’s going to be an important win if we get it because we’ll be a step closer to the playoffs,’’ Leerdam said. “We’re just fighting for a better position.’’

The Sounders sit four points behind Vancouver, tied with Portland and Sporting Kansas City, in a four-team battle for the conference crown. Sunday’s contest is their final road game of the season, followed by home matches in two weeks against a struggling FC Dallas squad and the Colorado Rapids after that.

They’ll likely have to do it without Jordan Morris, who continues to rehabilitate a badly strained hamstring. The Sounders hope that Morris — this week named No. 7 on the league’s “24 under 24” list of most talented players under age 24 (Cristian Roldan was at No. 13) — can return for the playoffs.

Leerdam hopes his squad can sustain some momentum from Wednesday, despite being short-handed without Morris and Rodriguez and even after the long, cross-country flight.

“We know we have some injuries now,’’ he said. “But everybody knows the game. Everybody knows how we have to play. We’ll try to move forward and see how it goes. It’s not an easy game because we’re on the road and we’ve traveled a lot.’’